Why Plants Evolved to be Green

why is green the dominant color of terrerestrial plants? That's easy. Chlorophyll molecules absorb light at the red and blue ends of the spectrum. It is the middle of the spectrum (the green part) that is reflected and gives plants their characteristic color. A more efficient photosynthetic pigment would be black, absorbing all colors, soaking up all the energy of sunlight, reflecting nothing. The reflected green light of plants is wasted energy. So perhaps the real question is. Why isn't grass black? Andrew Goldsworthy, a biologist at the Imperial College in London, has a possible answer. He draw^s our attention to certain purple bacteria that live in salt lakes, called Halobacterium haiobium. This particular microbe accomplishes a very primitive sort of photosynthesis using a pigment other than chlorophyll that absorbs light in the middle (green) part of the spectrum. This leaves red and blue light at opposite ends of the spectrum to be reflected and gives the bacteria their purple color. Goldsworthy believes this sort of microbe was the earliest form of life on Earth, teeming in great numbers in the earliest seas. Chlorophyll-using bacteria evolved later (according to Goldsworthy) and w^ere forced to make use of the light falling on the sea that wasn't already soaked up by the initially more numerous purple bacteria. These later, green bacteria were ultimately more successful, chemically speaking, and soon overwhelmed their purple cousins. And from the green bacteria evolved all green plants. Most of this is pure speculation on the part of Goldsworthy, but it raises the intriguing possibility that except for a fluke of evolution plants might have descended from H. haiobium, the purple photosynthesizer. In which case, we might today have purple grass.

An hypothesis that their microbial ancestors had to compete with purple lifeforms and green gave them a spectrum of sunlight to absorb.